The present invention relates to a method of measuring the refractive index of lenses and sample liquid. More particularly, the present invention relates to a method suitable for measuring the refractive indices and refractive index distribution of plastic lenses.
The present invention also relates to a matching fluid that is to be used in optical testing such as measurements of the refractive indices of lenses. The present invention also relates to an interferometer that employs said matching fluid or liquid.
The use of plastic lenses has expanded recently in order to meet various needs such as demands for lighter lenses, reduction in production cost and increased use of aspherical lenses. One big problem with plastic lenses is that they are not as stable as glass lenses in physical properties and that their refractive indices and distribution thereof will experience great variations during lens manufacture. It is therefore necessary to measure the refractive indices of individual plastic lenses and their distribution after molding. But these measurements should be nondestructive and hence are difficult to perform on an industrial scale. The difficulty is even greater in the case of aspherical lenses.
The refractive index of plastic lenses has conventionally been measured by the following procedure using an apparatus which is generally in the form of a Mach-Zehnder interferometer. Rays of light are used as reference light of plane waves, and a lens to be tested is set in the optical path of another set of rays, with the lens being immersed in a matching fluid whose refractive index is substantially equal to that of the lens under test. A glass sample with a known refractive index which is close to that of the lens under test is also set as a reference for refractive index measurement, with said glass sample being immersed in the same matching fluid. A test assembly comprising these components is placed in rays of light (serving as reference light of plane waves) in a Mach-Zehnder interferometer and the number of interference fingers produced is counted. The difference in sample thickness within the range where N interference fringes are observed is measured and the refractive index of the lens under test is determined on the basis of the measured value.
This method, however, has suffered from the problem that the refractive index of the matching fluid will vary with factors such as temperature variations. Thus, when temperature changes, the range within which N interference fringes appear will also change, making it necessary to perform another measurement of sample thickness in the affected area. A further problem is that nonuniformity in the refractive index distribution of a lens will produce irregularly shaped interference fringes, causing difficulty in obtaining the correct meaning of "N interference fringes". In other words, refractive index measurements under these circumstances have involved considerable difficulty in attaining reproducible results by quantitative interpretation. This has been a major cause of the difficulty encountered in attempts to automatic refractive index measurements.